A wavelength division multiplexing (WDM) scheme is a scheme of transmitting a plurality of optical signals having different wavelengths through a single optical fiber. A wavelength division multiplexing optical network is configured to include a central office (CO), optical network units (ONUs), and an optical distribution network that connects the central office (CO) with the optical network units (ONUs). As the most economic structure of several optical distribution networks, a star structure in which remote nodes (RNs) including wavelength multiplexer and demultiplexer are installed at adjacent areas of optical subscribers, the CO and the RNs are connected with each other through a single optical fiber, and the remote nodes and the optical subscribers are connected with each other through an optical fiber has been used.
The wavelength division multiplexing optical network structure that has been the most well-known up to now is a structure of dividing wavelength bands of upstream and downstream signals and using different wavelength bands. The wavelength division multiplexing network structure of the wavelength-band division type is most representative, in which single cyclic type wavelength multiplexer and demultiplexer are disposed at the central office and the remote nodes, respectively, the wavelength multiplexer and demultiplexer of the central office and the wavelength multiplexer and demultiplexer of the remote node are connected with each other through a single optical fiber, and each port of the wavelength multiplexer and demultiplexer and the optical network unit are connected with each other by a start structure. An optical transceiver used herein is configured of a wavelength-band dividing optical filter, an optical transmitter, and an optical receiver. As the wavelength-division band is narrow, it is difficult to technically implement the wavelength-band dividing optical filter. Therefore, when the upstream wavelength division multiplexing optical signal is used as a C-band wavelength band and the downstream wavelength division multiplexing optical signal is used as an L-band wavelength band, a method for dividing an optical module that wavelength-multiplexes the downstream wavelength division multiplexing optical signal and an optical module that wavelength-demultiplexes the upstream wavelength division multiplexing optical signal has been mainly used for the central office.
In the system apparatus of the wavelength division multiplexing optical network that has adopted a method for dividing the wavelength multiplexer and the wavelength demultiplexer according to the related art as described above, a method for controlling and monitoring the optical outputs and wavelengths of the optical transmitter of the optical network unit and the optical transmitter of the optical line terminal has been proposed. In the representative controlling and monitoring method, an output unit of the optical line terminal uses an optical power dividing tap coupler, an upstream signal controlling and monitoring wavelength demultiplexer, a downstream signal controlling and monitoring wavelength demultiplexer, a monitoring photodetector. The controlling and monitoring method needs to add two wavelength demultiplexers to the system apparatus of the wavelength division multiplexing optical network, and therefore the size and the construction cost of the system are increased.
In order to overcome the shortcomings, a method for installing a reflective tap coupler at the output unit of the wavelength multiplexer and installing a controlling and monitoring photodetector in front of the optical transmitter is used. However, a method for installing the controlling and monitoring photodetector in front of the optical transmitter requires a multi-wavelength reference signal generator and an expensive locking-Amp, and therefore the system construction has been complicated and the cost has increased.